Hi Alastair

The uplifted land had previously been covered with drift containing shells, forms and erratics from distant places. The uplift was not caused by an advancing terrestrial ice sheet as the evidence of thin marine ice deposition was already there.

i know the Dorset coast quite well and the Lulworth Crumple is just one of a series of complex structures in the area that could be caused by underlying salt of possible Permian or Triassic age. See Arkell's Geological Survey Memoir pp 295 - 296. etc. I agree it has nothing to do with the Alpine orogeny. However, see also :

Watts, A. B., McKerrow, W. S., & Fielding, E. (2000). Lithospheric flexure, uplift, and landscape evolution in south-central England. Journal of the Geological Society, 157(6), 1169-1177.

Watts, A. B. (2001). Isostasy and Flexure of the Lithosphere. Cambridge University Press.

Regards

Malcolm

Hi Norman,

It is nice to find out that I am not the only one who doubts that the Alpine Orogeny affected the Dorset coast.

Have you considered that an earlier glaciation could have laid down the drift? This would imply that later glaciations did not extend as far as earlier ones, but they could have been halted by the ridges created by earlier glaciations, or just not have advanced as far on some later fronts.

The density of rock is roughly twice that of ice, and it seems those ridges are less than half the height of ice sheets. So from an isostatic point of view the idea is feasible.

Would it be possible to date your thin marine depositions, and show that they are older than the last glaciation? But I will have to leave that with you as this is well beyond my capabilities and resources.

Cheers, Alastair.

Hi Alastair

I agree that it is likely that prior to a major phase of neotectonic uplift an earlier episode of continuing marine glaciation deposited the shelly drift. However there is no evidence that the later drumlin and moraine phases point to the presence of an ice sheet 600m to 1000m thick. Clearly the drumlins too were moved by continuing neotectonic disturbances of smaller magnitude after 14.000 BP and there is no evidence to support a thick mer de glace during the late Pleistocene.

Malcolm.

Hi Norman,

"The ice on both North America and Europe was about 3,000 m (9,800 ft) thick near the centers of maximum accumulation, but it tapered toward the glacier margins. Ice weight caused crustal subsidence which was greatest beneath the thickest accumulation of ice." https://en.wikipedia.org/wiki/Quaternary_glaciation

Glaciers have ice fronts, and I imagine ice sheets do too, so the front of the ice sheet creating the ridge could well have been 500m high, not 1m.

But I guess you are not going to buy my solution :-(

Cheers, Alastair.

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Regards…

Hi Hassam,

Alastair and I have very different views about continental ice sheets and the plausibility thereof. Balancing icesheet thickness with eustatic volume change of seawater is a circular argument. There is no self-evident facts that support such thick continental ice sheets. Also the Barbados coral evidence originally proposed by Fairbanks as evidence of lower eustatic sea level is suspect for two reasons: firstly, because Barbados is situated beside the Antilles subduction zone; secondly, because a recent global sea level drop of 120 metres would be recognised on seismic profiles of delta sequences throughout the world.

Changes in relative sea level are caused by rise and fall of continental crust driven by lateral flow of granite melt at the Conrad discontinuity. This is caused by neotectonic isostatic balancing of increasing sedimentary loading offshore and complementary uplift due to erosion onshore. There is abundant offshore sonar evidence to support this hypothesis.

Regards

NMM

Hi Norman,

I had not realised that you held such radical views!

The evidence for thick ice sheets exist today in Greenland (2 miles deep) and in Antarctica, (even thicker).

Paleo-sea level does not depend only on the Barbados coral reefs. Melt water pulse 1a is well documented. See https://en.wikipedia.org/wiki/File:Post-Glacial_Sea_Level.png

But the ice sheets had two conflicting effects on local sea level where the ice sheets were/are situated. The lower global sea level by storing water from the oceans on land, but also raise the local sea level by pressing the land under the weight of ice. Since the density of ice is half that of rocks than means that a two mile thick ice sheet will depress the land by one mile.

It makes me wonder whether Darwin was correct and the Roads of Glen Roy are raised beaches, produce at the height of the four major glaciations, rather than the shores of dammed glacial lakes that he later accepted.

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Hi Alastair,

Greenland is a weak reference area in your continent-wide, thick ice sheet, argument. This is because if you remove the ice, the underlying rock surface forms a saucer-shaped depression that will naturally fill with precipitation falling above the perennial snow-line, which declines from about 1000m at Cape Farewell to sea level at about 70 degrees north.

In spite of the mass of 3km of ice in central Greenland, only about a quarter of the area covered by that ice has been depressed below sea level, which raises doubts about any effectiveness of glacio-isostatic depression of the crust. (See attached map)

Further doubts are raised by the base of the Ice in Lake Vostok (central Antarctica), at 54 m above sea level, a fact that undermines glacio-isostasy still further.

You point to Meltwater pulse 1A as evidence supporting thick ice. It could equally well be the result of basin inversion of sub-regional extent ('Glacial lakes' Missoula and Agassiz) within a regional peneplain. Lake water above the (now evacuated) sedimentary basin would be spilled as a sludge to form the so-called 'glacial till' that extends across southern Canada, etc.

I spent 2 years trying to make glacio-isostasy and forebulges explain the sequence of topography-forming events apparent in South Donegal before I realised that there is no evidence for glaciers outside areas where mountains have been raised above the snow-line, and that eustasy is a specious circular argument.

I accept that these views may seem radical but they were commonplace to leading early Victorian geologists Humboldt, Darwin and Murchison amongst several others.